Oil-based inks and water-based inks have so far been used for writing instruments. In particular, ballpoint pens include ballpoint pens charged with oil-based inks and ballpoint pens charged with water-based inks. Among them, known are water-based inks referred to as a so-called gel ink which are prepared by providing water-based inks with a viscosity-reduced shearing property, and various proposals are made to a viscosity-reduced sharing property-donating agent therefor.
Proposed are, for example, application of xanthane gum (refer to, for example, a patent document 1), application of welan gum (refer to, for example, a patent document 2) and application of succinoglycan (refer to, for example, a patent document 3) as viscosity-reduced sharing property-donating agents.
However, in the ink compositions described in these respective patent documents, it is difficult to maintain dispersion and stabilization of the coloring agents over a long period of time, and ballpoint pens using these inks cause starving of drawn lines and bring about paling or darkening of lines when left standing in low temperature or high temperature atmosphere. Further, the dispersion system is broken, and the components in the inks are coagulated, whereby it has resulted in clogging at the pen tips to bring about the situation of impossibility to write. Writing instruments are used under various atmospheres, and therefore such deficiencies as described above are serious defects.    Patent document 1: Japanese Examined Patent Application Publication No. 8673/1989.    Patent document 2: Japanese Patent Application Laid-Open No. 214782/1992    Patent document 3: Japanese Patent Application Laid-Open No. 88050/1994